The present disclosure relates to a method for regulating a web tension in a processing machine.
Although the disclosure will be described in the following text substantially with reference to printing presses, it is not restricted to an application of this type, but rather can be used in all types of processing machines, in which a tension of a product web or material web is to be predefined. The product web can be configured from paper, material, cardboard, plastic, metal, rubber, in film form, etc.
In processing machines, in particular printing presses, a product web is moved along driven axles (web transport axles), such as pull rolls or feed rolls, and non-driven axles, such as deflecting, guiding, drying or cooling rolls. At the same time, the product web is processed by means of usually likewise driven processing axles, for example is printed, punched, cut, folded, etc.
The tension or web stress (as long as no cross-sectional change occurs, the tension and stress are proportional; however, the tension is usually measured) of the product web is influenced, for example, via what are known as clamping points which clamp the product web positively or non-positively. Here, these are regularly driven transporting or processing units. In a gravure printing press, a clamping point is usually formed by a printing unit, in which a frictional unit exists between the driven impression cylinder, the impression roller and the material web. The product web is divided into product-web sections, a product-web section being delimited by two clamping points. Further driven and/or non-driven axles can be arranged within a product-web section. The entire product web is often divided into a plurality of product-web sections, sometimes also with different setpoint tension values. In order to maintain the setpoint values, what is known as web stress regulation (web tension regulation) is usually used. The regulation of the web tension usually takes place via a strain as manipulated variable, by the rotational speed of the clamping points being influenced.
The regulation of the web tension of a product-web section can take place in different ways. Downstream means that the clamping point which delimits the product-web section downstream is adjusted, and upstream means that the clamping point which delimits the product-web section upstream is adjusted. In this simple embodiment, however, the web tension in preceding and/or following product-web sections is not decoupled by the actuating movement. Rather, the change in the web tension is transported through the machine so as to follow the product-web course and is to be adjusted in all following sections. In addition to this indirect disruption on account of the transport of the material web, a direct disruption on account of the actuating movement is to be found in the product-web section which adjoins the adjusted clamping point.
It is possible, in the case of downstream regulation, to pilot control all the following clamping points by means of (dynamic) downstream pilot control, in such a way that said following clamping points compensate directly for the effects of the preceding clamping point, that is to say the web tension does not change there. As a consequence, it is ensured that all the following web-tension regulators do not have to compensate for the disruptions of the actuating movement and of the coupling by the material web. This is described, for example, in the publication “Simulation and Optimierung des Bahnspannungsverhaltens” [Simulation and optimization of the web-stress behavior], 9th web running seminar of the Technical University of Chemnitz, Chemnitz, 2007, Schnabel, H., Dörsam, E., Schultze, S. The web tension in the following product-web sections is decoupled here from the regulation of the web stress in the preceding product-web section.
DE 10 2008 056 132 A1 proposes decoupling for upstream regulation, (dynamic) downstream pilot control by means of PT1 element being carried out in addition to (constant) upstream pilot control.
DE 10 2009 016 206 A1 discloses a method with decoupling which is implemented exclusively in the upstream direction. Here, a combination is disclosed comprising pilot control which is weighted in the upstream direction by a DT1 element and pilot control, which is weighted by a negative PT1 element, exclusively of the rear delimiting clamping point. The specifications “upstream of” and “downstream of” a clamping point or a product-web section relate to the transport direction of the product web, that is to say the product-web course.
However, machine configurations are then also possible, in which a web stress change from one product-web section is reflected only after a time delay in another product-web section, the region between the two sections being unregulated and being called a delay section in the following text. The time delay is also called dead time.
A delay section is formed, for example, if the product web wraps around one or more rolls, such as cooling or drying rolls, the web tension approximately not changing during the roll contact as a result of the snug fit of the web against the roll. No concrete solution is specified in the prior art for cases of this type.